This invention relates to a process for manufacturing phosphoric acid, the starting material being the mother liquor from the Odda process after first filtering off the precipitated calcium nitrate tetrahydrate. More particularly this invention relates to a process for manufacturing concentrated phosphoric acid containing calcium phosphates, produced by evaporating Ca-containing mother liquor and thereby removing the nitrate and fluorine present in the mother liquor in the form of nitric acid, hydrofluoric acid and silicon tetrafluoride.
In the Odda process the lime and the phosphoric acid in the phosphate rock are converted to soluble form by nitric acid acidulation of the phosphate rock. The major part of the calcium present is crystallized by cooling in the form of calcium nitrate tetrahydrate, and the crystals are removed from the solution e.g. by means of filtration.
The mother liquor may be further processed in different ways. One way is to make phosphoric acid by removing the nitric acid by evaporation, whereas the residual calcium is precipitated as sulphate and removed. The direct evaporation of the nitrate components and further utilisation of the phosphoric acid-containing residue is a prior known process described in Norwegian Patent No. 50185 (corresponding to U.S. Pat. No. 1,834,455) taken out by Erling Johnson, the inventor of the Odda process. According to this patent the nitric acid in the mother liquor is removed by evaporation, while the calcium residue is precipitated as sulphate. According to the patent, precipitation can be carried out prior to or after removal of the nitric acid by evaporation.
However, due to specific corrosion problems and build-up of deposits in the equipment the process has not been technically feasible. A particular problem has been the removal of the nitric acid when the mother liquor contains substantial amounts of calcium. In spite of the fact that the Odda process has been known for about 40 years, during which time it has been considered to be desirable to establish commercial production of phosphoric acid by this process, these technical problems have been so serious that up to now there has been no suitable technical solution found which is applicable on an industrial scale.
An effort was made in the early 1960's to introduce an industrial process for the manufacture of phosphoric acid based on the Odda process mother liquor. This process is described in the article "Improved route to phosphoric acid" - Chem. Eng. Prog., Vol. 62, No. 2, February, 1966.
The process attempted to solve the specific corrosion, scaling and engineering problems by more extensive removal of calcium and other harmful contaminants from the mother liquor before evaporating the nitrate (nitric acid). The fluorine contaminant was sought to be removed by the addition of finely divided silica (SiO.sub.2), and stripping off in the nitric acid acidulation stage, reaction being carried out under boiling, whereby nitric acid-water-SiF.sub.4 vapour was formed which was removed from the reaction mixture as it was formed - see U.S. Pat. No. 3,205,062. Further it was also attempted to remove dissolved nitrates by the addition of barium carbonate, the barium nitrate being precipitated and removed from the liquor -- see U.S. Pat. No. 3,195,980 -- the calcium residue being removed by means of the sulphuric acid addition and precipitation as gypsum. It was not until after the completion of these process stages that final evaporation of the nitric acid was conducted.
However, even this seemingly technically advanced process has not been a success. Use of barium carbonate to precipitate soluble nitrate residue is very costly, and environmental problems arise if sulphuric acid is used for complete removal of the calcium content.